You Can't Get Pregnant While Menstruating
Couples who are trying to avoid a pregnancy but want to take a break from using contraception might think that having sex during a woman's period gives them a "get out of baby-making free" pass. While this method of avoiding conception may help reduce pregnancy odds, it is by no means foolproof.
In fact, a study conducted by the National Institute of Health Sciences (part of the National Institutes of Health) has shown that there are very few "safe" days in which sex without pregnancy can occur. The research reported that if you count the day on which a woman begins her period as day one, she could be fertile again by day four. It also showed that even among women with regular cycles, 1 to 6 percent could be fertile on the day their next menstrual cycle is due to begin [source: Grigg].
Combine this with the fact that sperm from ejaculation can live inside a woman's body for up to five days, and it soon becomes clear that there are no free passes in the baby-making game [source: Harms].